A variety of valve operation controllers have been conventionally proposed for variably controlling opening/closing timings and lift amounts of an intake valve and an exhaust valve in order to appropriately tuning the intake/exhaust performance in accordance with an operating state of an internal combustion engine. As one of such conventional valve operation controllers, there has been known a type of controller which changes the phase of an intake cam (hereinafter called the “cam phase”) for a cam shaft (see, for example, Laid-open Japanese Patent Application No. 7-301144). This type of valve operation controller continuously changes the valve phase to continuously control opening/closing timings of an intake valve. For example, when an engine is heavily loaded, the valve operation controller continuously changes the opening/closing timings of the intake valve in accordance with the rotational speed of the engine to maximally utilize the inertia effect of intake air to enhance a filling efficiency, thereby increasing the power. On the other hand, when the engine is lightly loaded, the valve operation controller advances the opening/closing timings of the intake valve to extend a valve overlap with an exhaust valve (a period in which both valves are open) to increase the amount of internal EGR, thereby decreasing the combustion temperature to reduce the amount of emitted NOx.
As another type of conventional valve operation controller, there has been known a valve operation controller which has a lower speed cam and a higher speed cam having predetermined cam profiles different from each other for each of an intake cam and an exhaust cam for switching the cam profile of each cam between the lower speed cam and higher speed cam (see for example Laid-open Japanese Patent Application No. 62-12811). This valve operation controller switches the intake and exhaust cams to the lower speed cams during low rotational speed and to the higher speed cams during high rotational speed to provide optimal opening/closing timings and lift amounts for the intake and exhaust valves in each of the rotational speed ranges to ensure a high intake/exhaust efficiency, thereby realizing high engine performance. Another known valve operation controller of this type sets a cam profile of one of intake cams to open/close an intake valve by a minute lift amount to generate a swirl so that a stable combustion state is ensured even with a lean air/fuel mixture, thereby improving the fuel efficiency and eliminating fuel stagnation in the intake valve (see, for example, Laid-open Japanese Patent Application No. 7-97971).
A further known valve operation controller of another type employs electromagnets to open/close an intake valve and an exhaust valve (see for example Laid-open Japanese Patent Application No. 8-200025). This valve operation controller comprises main and auxiliary intake valves and main and auxiliary exhaust valves in each cylinder, and electromagnetic valve operating mechanisms each for driving associated one of these four intake and exhaust valves. Each electromagnetic valve operating mechanism comprises two electromagnets opposing to each other; an armature arranged between the two electromagnets and coupled to an intake and exhaust valves corresponding thereto; two coil springs for urging the armature; and the like. This electromagnetic valve operating mechanism controls the conduction of both electromagnets to alternately attract the armature to the respective electromagnets to open/close the intake and exhaust valves. Therefore, the valve opening timing and valve closing timing can be arbitrarily controlled for the intake and exhaust valves by controlling the conduction timing. When both electromagnets are not conductive, the armature is held at a neutral position between both electromagnets by a balance of urging forces of both coil springs, thereby holding the intake and exhaust valves in an open state.
Also, in this valve operation controller, the main intake valve and auxiliary intake vale are driven in a different combination in accordance with a particular operating state of the internal combustion engine. Specifically, the main intake valve is paused and the auxiliary intake valve only is operated in a low-speed low-load state; the auxiliary intake vale is paused and the main intake valve is operated in a middle-rotation middle-load state to supply an amount of intake air suitable for the operating state, while generating a swirl, to ensure a stable combustion state; and both main and auxiliary intake valves are operated in a high-rotation high-load state to ensure high power.
However, among the conventional valve operation controllers described above, the first type one which varies the cam phase simply changes the phase of the intake cam with respect to the cam shaft. Since a valve opening period is fixed for the intake valve, the valve closing timing is automatically determined as a valve opening timing is set for the intake valve. For this reason, the valve operation controller cannot simultaneously provide an optimal valve opening timing and an optimal valve closing timing in the overall rotational speed region and load region. For example, while it is preferred that valve opening and closing timings are set to minimize the fuel efficiency at a combustion fluctuation limit during a low speed operation and to maximize a torque during a middle to high speed operation, this valve operation controller encounters difficulties in realizing such valve operations.
The second type of valve operation controller which switches cam profiles only has two stages of cam profiles to switch, so that the opening/closing timings and lift amounts provided thereby for the intake and exhaust valves merely change in two stages. Therefore, the valve operation controller fails to provide optimal opening/closing timings and lift amounts in the overall rotational speed region and load region.
On the other hand, the third type of conventional valve operation controller having the electromagnetic valve operating mechanisms is capable of arbitrarily controlling valve opening timings and valve closing timings for the intake and exhaust valves, so that this valve operation controller can advantageously provide optimal opening/closing timings in the overall rotational speed region and load region. However, since this valve operation controller drives all the intake and exhaust valves using the electromagnetic valve operating mechanisms, the power consumption is increased, causing a resulting reduction in the fuel efficiency. Also, since the electromagnets, armature and the like, forming part of the electromagnetic valve operating mechanisms, are made of magnetic materials, the valve operation controller is disadvantageous in a larger weight and an increased manufacturing cost.
Also, since this valve operation controller relies on the attraction of the armature by the two electromagnets to open/close the intake valve, the lift amount for the intake valve is fixed, so that difficulties would be encountered in opening/closing the intake valve by a minute lift amount. Therefore, when the main intake valve or auxiliary intake valve is paused, it must be held in a fully closed state. Depending on a particular operating condition of the internal combustion engine, if this main or auxiliary intake valve is continuously held in the fully closed state, carbons generated by combustion may stick to the paused intake valve and a valve seat thereof, in which case the intake valve is forcedly torn off from the valve seat when the intake valve resumes the operation, thereby possibly damaging the sealability with the valve seat. Also, since the paused intake valve in the fully closed state results in fuel stagnation, an air/fuel mixture is enriched to degrade the exhaust gas characteristic when the intake valve resumes the operation.
Further, in this valve operation controller, when the armature is applied with a mechanical impact or some vibrations when the armature is being attracted by an electromagnet or is moving between the electromagnets, the valve operation controller can suffer from a phenomenon in which the intake and exhaust valves are returned to their neutral positions by urging forces of the coil springs (hereinafter called the “step-out phenomenon”). Particularly when the step-out phenomenon appears on the exhaust valve side, an unburnt gas is exhausted to the outside through the stepped-out exhaust valve during a compression stroke and an explosion stroke, thereby possibly resulting in degraded exhaust gas characteristic.